Abstract

Aquatically foraging Bewick’s Swans Cygnus bewickii have been repeatedly
reported to be accompanied by diving ducks, but the exact nature of this relationship
is unclear. Based on field observations, we found a strong correlation
between the number of foraging swans and the number of foraging Common
Pochards Aythya ferina and Tufted Ducks Aythya fuligula at our study site
(Lauwersmeer, the Netherlands), with the median ratio being close to one
diving duck per swan. To determine whether the association was kleptoparasitic
or commensal, we measured in a series of experimental trials the instantaneous
intake rates of Bewick’s Swans and Common Pochards when foraging
alone and when foraging together. On average, pochards in company with a
swan attained twice the instantaneous intake rate as compared with foraging
alone. This was mainly due to gleaning food items that floated away as a result
of the swan’s trampling activity, and was not due to stealing: in fact, food intake
rates of swans were not affected by the presence of a pochard.